Molybdenum chemistry in molten LiCl–KCl eutectic: an electrochemical and absorption spectroscopy study of the concentration dependent stability of solutions of K3MoCl6

Abstract The chemical behaviour of LiCl–KCl–K 3 MoCl 6 solutions between 400 and 500°C was investigated. They underwent slow decomposition, according to a concentration dependent disproportionation reaction yielding metallic molybdenum and a volatile black higher valency molybdenum compound, which corresponded with MoCl 5 . Chronopotentiometric determinations showed a single 3-electron reduction, yielding Mo. The process was slow with diffusion-controlled mass transfer. The concentration of the Mo(III) species, presumed in the form of MoCl 6 3− ions, was determined using linear sweep voltametry and open circuit voltage measurements and the overall Mo(III) species concentration by chemical analysis. The results showed MoCl 6 3− to be the predominant species in the melt and the presence of a previously proposed dinuclear species, Mo 2 Cl 9 3− , could not be confirmed electrochemically. This arose to quantification errors in the determination of the electroactive species MoCl 6 3− due to inherent problems in estimating the active surface area of the electrode. The electronic absorption spectrum of MoCl 6 3− in LiCl–KCl eutectic at 400°C is reported for the first time. Measurements were made above and below 0.045 mol dm −3 K 3 MoCl 6 , the concentration above which the disproportionation reaction was relatively slow. The results are discussed in detail and the spectroscopic parameters of the octahedral MoCl 6 3− complex were calculated as: Dq=1775 cm −1 , B =511 cm −1 and C =1913 cm −1 . Although an additional band, at 340 nm, appeared during disproportionation it could not be identified with any currently known polynuclear molybdenum chloro complex or oxychloro complex. It is probably associated with an intermediate in the disproportionation process. No spectral evidence was thus obtained in support of the formation of dinuclear molybdenum species.